This invention relates to a device for manufacturing by indirect extrusion laminated semifinished material from a layered sinter-bond blank, the layers of which blank consist of metal, metal alloys, metal mixtures or mixtures of metals and nonmetallic substances. The indirect extrusion device comprises a vessel with a rectangular cutout and an extrusion die of one or more parts with one or more cutouts.
Contacts for low-voltage air switchgear usually consist of silver base materials to which metaloids or metal oxides are added for reducing the welding power and for improving the burnoff behavior. These material additives make the formation of a welded or soldered bond with a support part more difficult. For this reason, the contacts are frequently provided with a second solderable or weldable layer.
If contacts are fabricated by initially extruding single-layer strips from sintered blanks as semifinished material, the initially single-layer extrusions must be coated, after suitable and often elaborate pretreatment, with the solderable layer and subsequently cut into contacts. Thus, as described in German Open Patent Application (Deutsche Offenlegungsschrift) No. 2,848,980, an auxiliary plating layer may be applied and subsequently plated by hot rolling and rolled to proper dimensions. Especially with distinctly brittle materials such as AgSnO.sup.2, the use of which materials facilitate control of environment pollution, difficulties arise in plating by hot rolling, with the result that the usual oxide contents are limited to 10 to 12% by weight because of decreasing deformability with increasing oxide contents. To improve the deformation properties of materials, the starting powders are subjected to additional annealing and granulating treatments, as described in German Open Patent Application (Deutsche Offenlegungsschrift) No. 2,952,128.
It has furthermore been attempted to produce plating during the extrusion process by simultaneously extruding the contact material with the metal of the solderable layer. British Pat. No. 880,583 described a method which images the layer geometry of the blank in the extrusion to scale by extruding a multilayer sintered-bond part. In fabricating the blank, a support web is inserted into the powder pressing die in such a manner that it represents a scale image of the desired boundary layer geometry of the extrusion. The powder chambers so produced are filled to the same height with respective powders, the partition is removed and the powders are pressed together in a pressure direction parallel to the powder boundary layer. The pressed part produced in this manner is subsequently sintered and then extruded, the direction of pressure being again parallel to the boundary layer. Strength problems at the boundary layer arise here due to the usually different densifiabilities of the different powders.
As disclosed in German Patent Document (Deutsche Auslegeschrift) No. 1,539,848, the strength of boundary layers produced by this "imaging" method can be improved if the partition inserted into the powder pressing tool is serrated at the bottom, so that the powders are mixed at the boundary layer when this partition is pulled out. This mixture of the powders, however, leads to an undesirably wider transition zone between the different materials.
Both these methods (the rolling method and the "imaging" method) use the direct extruding process, usually called "extruding", in which the batch to be extruded is pushed through the tool in the forward direction, i.e., in the direction of motion of the pressing element. The friction force between the extruded material and the vessel wall, which initially can be up to twice the deformation force proper, is reduced considerably with the shortening of the remaining batch, with the consequence that the layer geometry and the layer thickness shift or change over the length of the extrusion. Because the friction conditions are different from extrusion to extrusion, this effect cannot be compensated by making the boundary layer in the pressed powder part wedge-shaped. A further disadvantage of direct extrusion is the large amount of return material, which is in general more than 10% ("Metall" vol. 36 (1982) no. 4, pages 439 to 443). This ratio is further increased in the extrusion of laminated sintered-bond parts because at the start of the extrusion a considerable running-in region occurs until the different layers flow to the die opening and instantaneous equilibrium conditions can settle there. The equilibrium, however, is shifted again over the length of the extrusion with the change of wall friction. In this direct extrusion method, only a material discharge of less than 70% can be expected.
These known methods have not found acceptance because of problems with layer thickness tolerance, strength of the boundary layers and lower yield.
The two-layer powder pressing technique for manufacturing finished formed parts has proven successful in solving these problems. In this technique laminated parts comprising, for instance, one layer of contact material and another layer of solderable material are produced by filling a compacting chamber with the powder layers one on top of the other and subsequently pressing the layers together. The direction of the pressure is in this case perpendicular to the boundary layer so that different densification behavior of the powders during the pressing has no disturbing effect on the strength of the boundary layer (see "Pulvermetallurgie elektrischer Kontakte" ("Powder Metallurgy of Electrical Contacts"), Springer-Verlag, Berlin 1964, pages 211 to 213).
This two-layer powder pressing method is limited to small parts and, in particular, does not allow the use of high degrees of deformation for densifying and increasing the strength of the pressed powder parts, which limitation leads to special vulnerability of the two-layered materials to mechanical, thermal and electrical stresses.
An object of the present invention to develop an improved device for producing, in a simple manner and without additional plating operations, laminated semifinished material by indirect extrusion of a sintered-bond blank having layers of metal, metal alloys, metal mixtures or mixtures of metallic and nonmetallic substances. Another object of this invention is to produce such a device in which extruded multilayered materials have a layer thickness uniform over the length and the cross section of the extrusion and in which there is great adhesion of the layers to each other, as well as high material utilization.